Luminescent concentrator and adaptation thereof in a window, facade and other building elements.
The present invention relates to a panel-shaped con¬ centrator for concentration and conversion of solar radiation and"artificial light into electrical energy, heat or for transfer thereof as light to a point of application. A panel-shaped concentrator is provided with radiation absorbing luminence centers which re-emit the radiation in another wavelength range.
Conversion of solar energy directly into electrical• energy is in principle possible by means of solar cells. Such cells represent high-level technology which also means that manufacturing costs will be remarkable. Thus, the aim in exploitation thereof is to maximize the output of said cells. -Also, use •is made of cheapter materials and less sophisticated manufacturing methods, although at the cost of efficiency, A third alternative is to exploit sunlight as effectively as possible by concentrating and focusing radiation on a solar cell by means of an array of reflectors and lenses. Thus, the share of costs induced by solar cells is reduced, facilitating the use of high-quality expensive solar cells. However, this type of system includes a complicated and expensive sun tracking mechanism.
An object of the present invention it to provide a simple system for effective exploitation of solar radiation, including diffuse radiation, said system also eliminat¬ ing the need of a tracking mechanism. An apparatus of the invention is characterized in that a panel- shaped element is provided or coated with, radiation absorbing luminence . centers for re-emitting the radiation within another wavelength range, and that the emitted radiation is concentrated by means of total reflection, element design and/or coating on the radiation receiving part or parts of said panel- shaped element.
One of the advantages of a luminescent concentrator over other focusing systems is its capability of utilizing also diffused radiation which,e.g. in Fin¬ land's conditions, makes up approximately half of total radiation, as well as its operation without an expensive tracking mechanism. A radiation receiving means may be any prior known element, such as a solar cell or an optical fiber means or an absorption surf¬ ace or a like element in which radiation is converted into heat.
Since the emitted radiation arriving at a receiving means is of narrow band, it is possible to select a solar cell having optimum spectral characteristics relative thereto and thus operating at higher effic¬ iency.
Also possible are multi-layer designs for reducing losses and improving the system efficiency. The manufacturing process of a panel-shaped concentrator is simple and economical and, by virtue of concentrat¬ ion function, the price of solar cells is only a minor part of the total costs of the system.
The applications of a panel-shaped concentrator of the invention include e.g. roofs, windows and other surf¬ aces of buildings as well as protective glazings of
CMPI solar collectors.
As pointed out above, emitted in a concentrator is relatively narrow-band radiation, whereby it is possible to select for each application the most preferable solar cell'for increased efficiency. Thus, it will be possible to employ solar cells whose spectral characteristics are optimized for a certain type of concentrators or it will also be possible to employ various types of solar cells for absorbing various wavelength ranges. Since the wavelength ranges unfit for a given application have been filter¬ ed away, no waste heat or undesirable heating of a solar cell will be produced. There are available even today mass-production plants which can be operated for manufacturing panel-shaped concentrators of the in¬ vention. Furthermore, materials are not particularly expensive.
A radiation concentrated on a radiation receiving means can be converted into heat or electrical energy and, if such means is comprised of optical fibers, the concent¬ rated light is passed to a point of application.
In its most simple embodiment, a concentrator comprises a radiation permeable panel in which luminescent matter is distributed as a layer inside the panel or as a coating on the surface thereof.
A luminescent material usable for the application com¬ prises e.g. dyes used for dye lasers as well as rare earth metals. An optimum luminescent material has a wide absorption range covering most of the solar spectrum as well as a narrow emission range having a somewhat longer wavelength. Thus, the luminescent material absorbs incoming solar radiation and emits it at another wavelength. In this process, efficiency is very high, with some dyes nearly 100 %. Emission runs in all directions and those rays that meet the interface with an angle greater than the critical cone of total reflection remain inside the panel. It depends on the refraction factor of a panel how much of the radiation can be recovered in the panel. Approximat¬ ely 75 % of the radiation can be recovered in glass whose refraction factor is about 1,5. The share of recoverable radiation can also be increased by means of a reflective coating on one surface of a concentrator or by means of a selective coating which reflects the emitted radiation but is permeable to the radiation to be collected. Thus, the radiation in a panel or plate can be concentrated e.g. in the edges of a panel. In order that the radiation would still find its way to the edge regions, neither collector material nor luminescent material may absorb emission radiation, in other words, absorption and emission spectra must be kept away from each other.
The efficiency of a panel-shaped concentrator of the invention can be increased by employing a plurality of concentrators combined together. Thus, the solar spectrum can be subdivided into a plurality of wave¬ length bands and, according to said bands, also solar cells can be selected properly for each con¬ centrator. With this combined concentrator, it is also possible to utlize and recover in a subsequent concentrator the losses possibly occureed in a pre¬ ceding concentrator.
The design of a concentrator can be varied according to whether it is desired to employ radiation receiving means on just one edge or several edges of a concent¬ rator or whether radiation receiving means are. to be positioned inside a concentrator. In the latter case, the design of a collector panel has not a major practical significance. In the former case, those edges not provided with radiation receiving means are made reflective e.g. by using a metal foil or some other reflective coating. Such a metal reflection results in a certain loss and thus, in view of toal efficiency, it is preferable to maintain the number of such reflections as low as possible. In this respect, a preferred design a rect¬ angular isosceles triangle in which the em ited rad¬ iation is collected on the edge region of hypotenuse, the result being just two reflections.
For reducing the reflection losses occurring on the surface of a concentrator as well as between its edge and a solar cell, the surfaces can be preferably coated with anti-reflective coating.
The concentrator or collector material is e.g. glass or poly ethylmetacrylate (plexiglass or other suitable plastics) . A luminescent material preferably used in connection with glass comprises rare earth metals, e.g. neodymium. When the actual raw material is plastics, luminescent materials employed are preferably organic dyes. A plurality of luminescent or fluorescent materials can be included in a single concentrator according to application.
A panel-shaped concentrator according to the invention can be used as a window structure of a building.
A panel-shaped concentrator can also be used in facade or roof structures and also in building elements and systems intended for these applications.
Other applications include protective glazings of con¬ ventional solar collectors and individual or extensive solar collector systems based on the use of a concentrator of the invention.